\(\theta _n^0\)  is the angular position of \(\alpha =2n\pi \) . In the limit
\(n\longrightarrow \infty \) , the relation between the minimum of impact
parameter \(u_{ps}\)  and asymptotic position of a set of images
\(\theta _{\infty }\)  can be expressed by
\(u_{ps}=D_{OL}\theta _{\infty }\) . In order to obtain the coefficients
\(\bar{a}\)  and \(\bar{b}\) , in the simplest case, we separate the
outermost image \(\theta _1\)  and all the remaining ones which are
packed together at \(\theta _{\infty }\) , as done in Refs
[1]}, [2]}. Thus \(s=\theta _1-\theta _{\infty }\)  is
considered as the angular separation between the first image and
other ones and the ratio of the flux of them is given by,
\(\mathcal {R}=\frac{\mu _1}{\sum _{n=2}^{\infty }\mu _n}.\) 
